86 GN does have the 8.5 inch rear... a cutlass 442 also has an 8.5 that will bolt right in... i would check in the parts for sale section and maybe there is a rear in there for you... or post in the parts wanted.. if you are running real fast you could go with a ford 9", but that takes a lot of work, be careful though, if you go to a bone yard, most G-bodies are 7.5" rear ends, and boy you really dont want one of those....
-neil